The "M" is a special operations designator (MH-60G. MH-60K, MH-47, MC-130H, etc...)
The "H" is a rescue designator (HH-60G, HH-60H, HH-65, HC-130P, etc...)
The "L" varient -60's were an upgrade of "A" models or new builds which among other things included new engines (T700-GE-701C) with more shaft horsepower (1843shp) compared to T700-GE-700 in the UH-60A producing 1622shp per engine. HIRSS was also added. There's many more upgrades in the "L" model but I'm not going to list them all ('cause I'm sure I'd forget a few) .
I'm not really sure what the "S" designator is for in Navy -60's but I think it's anti-submarine???
BTW, MRC is Academy right???